1. Field of the Invention
The present invention relates to a de-spreading device applicable to a direct sequence spread spectrum (DS-SS) communication system and operable in a power-saving mode, and a method for the same.
2. Description of the Background Art
It is a common practice with a demodulator for a spread spectrum communication system to compare a signal output from a detector with a preselected threshold value to generate bi-level virtual chip data, which are of (logical) ZERO or (logical) ONE, to in turn execute correlation operation on the virtual chip data by use of a correlator, and then, based on the peak value of a correlation value, to estimate the optimum timing of symbol data decision and detect symbol data.
Japanese patent laid-open publication No. 2003-283369, for example, discloses a correlation detector configured to use a replica spread code to de-spread a digital spread-spectrum received signal with a correlator and then demodulate the resulting signal with a demodulator. More specifically, a matched filter produces a correlation from a digital spread spectrum code sequence while a correlation peak detector detects a correlation peak value out of the above correlation. Subsequently, a synchronous tracing circuit determines, on the basis of the correlation peak value, a timing for generating a replica spread code. In response to the timing thus determined, a replica spread code generator generates a replica spread code. Particularly, the matched filter includes bit-based filter segments each implemented by an Exclusive OR (EX-OR) gate and an adder.
The correlation detector taught in the document mentioned above compares demodulated data not subjected to error correction with demodulated data subjected to error correction bit by bit to thereby determine the number of bit errors, calculates, on the basis of the number of bit errors, a bit error rate with a receipt quality decision circuit, compares the bit error rate thus determined with a preselected threshold value to thereby determine receipt quality, and then selects, based on the determined receipt quality, the bit-based filter portions to be enabled by a search controller. When receipt quality is high, the correlation detector enables only a minimum necessary number of bit-based filter portions capable of detecting a correlation peak higher than the current correlation peak. This condition is held so long as receipt quality is high. It is therefore possible to reduce power consumption by the matched filter, which inherently consumes much more power, to a minimum necessary level.
Generally, the demodulation characteristic of a demodulator can be enhanced if the sampling frequency of a correlator is increased to improve the accuracy of timing for symbol data decision for thereby reducing symbol data decision errors. With such a correlator, a significantly high sampling frequency may not be required in a high C/N (Carrier-to-Noise) ratio environment. In a low C/N ratio environment, however, the sampling rate must be increased to some extent in order to achieve a desired characteristic. In this respect, a conventional demodulator has the following problem left unsolved.
In a conventional demodulator, an operational clock signal input to a correlator has its frequency fixed. The operational clock signal is therefore usually so determined as to satisfy a desired characteristic even when radio channel quality is low, e.g. when sensitivity is at the lowest limit that should be guaranteed. It follows that when radio channel quality is high a demodulator adapted for operating with a preselected operational clock signal without regard to radio channel quality wastefully consumes excessive current.